test(member-live): keep deadlock-prone member tests synchronous

These member/group/custom-field LiveView tests stay async: false. With the
foreign keys now deferrable the create_member deadlock no longer forces it, so
the rationale is updated: they remain synchronous as a deferred scope decision,
and index_groups_url_params/member_filter_component additionally have separate
async-isolation issues that must be fixed before they can run in parallel.

test/mv_web/member_live/show_groups_display_test.exs

@@ -10,8 +10,10 @@ defmodule MvWeb.MemberLive.ShowGroupsDisplayTest do
   - Accessibility: group links have aria-label for screen readers
 
   ## Note on async
-  async: false to avoid PostgreSQL deadlocks when creating members and groups
-  in the same test run (same as IndexGroupsDisplayTest).
+  Kept async: false as a deferred scope decision. The deferrable-FK migration
+  removed the concurrent-create_member deadlock that previously forced this, so
+  re-flipping this members/groups suite to async is a possible follow-up rather
+  than part of the original change.
   """
   use MvWeb.ConnCase, async: false
   import Phoenix.LiveViewTest